 //获得的是元素
 let app = document.getElementById('app')
 let nextElement = app.nextElementSibling
 console.log(nextElement)
 let prevElement = app.previousElementSibling
 console.log(prevElement)
 let firstElemnt = app.firstElementChild
 console.log(firstElemnt)
 let lastElemnt = app.lastElementChild
 console.log(lastElemnt)
 //假如array是一个数组
 let array = [1, 2, 3]
 //array = null
 let arr = array || []
 console.log(arr)